shmctl(2)

NAME

     shmctl - shared memory control operations

SYNOPSIS

     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION

     The shmctl() function provides a variety  of  shared  memory
     control  operations  as  specified  by  cmd.  The permission
     required for a shared memory control operation is  given  as
     {token},  where  token is the type of permission needed. The
     types of permission are interpreted as follows:

     00400    READ by user
     00200    WRITE by user
     00040    READ by group
     00020    WRITE by group
     00004    READ by others
     00002    WRITE by others

     See the  Shared  Memory  Operation  Permissions  section  of
     intro(2) for more information.

     The following operations require the specified tokens:

     IPC_STAT
           Place the current value of each  member  of  the  data
           structure  associated  with  shmid  into the structure
           pointed to by buf. The contents of this structure  are
           defined in intro(2). {READ}

     IPC_SET
           Set the value of the following  members  of  the  data
           structure  associated  with shmid to the corresponding
           value found in the structure pointed to by buf:

           shm_perm.uid
           shm_perm.gid
           shm_perm.mode  /* access permission bits only */

           This command can be executed only by  a  process  that
           has  an effective user ID equal to that of super-user,
           or to the value of shm_perm.cuid or   shm_perm.uid  in
           the data structure associated with shmid.

     IPC_RMID
           Remove the shared memory identifier specified by shmid
           from  the system and destroy the shared memory segment
           and data structure associated with  it.  This  command
           can  be  executed only by a process that has an effec-
           tive user ID equal to that of super-user,  or  to  the
           value  of  shm_perm.cuid  or  shm_perm.uid in the data
           structure associated with shmid.

     SHM_LOCK
           Lock the shared memory segment specified by  shmid  in
           memory. This command can be executed only by a process
           that has an effective user ID equal to super-user.

     SHM_UNLOCK
           Unlock the shared memory segment specified  by  shmid.
           This  command  can  be executed only by a process that
           has an effective user ID equal to super-user.

     Shared memory segments must be explicitly removed after  the
     last reference to them has been removed.

RETURN VALUES

     Upon successful completion, 0 is returned. Otherwise,  -1 is
     returned and errno is set to indicate the error.

ERRORS

     The shmctl() function will fail if:

     EACCES
           The cmd argument  is  equal  to  IPC_STAT  and  {READ}
           operation permission is denied to the calling process.

     EFAULT
           The buf argument points to an illegal address.

     EINVAL
           The shmid argument is not a valid shared memory  iden-
           tifier;  or the cmd argument is not a valid command or
           is IPC_SET and shm_perm.uid  or  shm_perm.gid  is  not
           valid.

     ENOMEM
           The cmd argument is equal to SHM_LOCK and there is not
           enough memory.

     EOVERFLOW
           The cmd argument is IPC_STAT and uid  or  gid  is  too
           large to be stored in the structure pointed to by buf.

     EPERM The cmd argument is equal to IPC_RMID or  IPC_SET  and
           the  effective  user  ID of the calling process is not
           super-user and  it  is  not  equal  to  the  value  of
           shm_perm.cuid  or  shm_perm.uid  in the data structure
           associated with shmid.

     EPERM The cmd argument is equal to  SHM_LOCK  or  SHM_UNLOCK
           and  the  effective  user ID of the calling process is
           not equal to that of super-user.

SEE ALSO

     ipcs(1), intro(2), shmget(2), shmop(2)